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xv                                 SKULL   OF   RODENTS                              461
encircling ridge is broken up into tubercles, which gives to the
tooth a striking likeness to those of Ornithorhynchus. Other
genera have teeth like those of many Ungulates. It has been
shown by Sir J. Tomes1 that the minute structure of the enamel
differs in different groups of Rodents.
The skull shows certain primitive characters. In the first
place there is no distinction between the orbital and the temporal
fossa.2 The sutures between the bones retain their distinctness
for very long. Other characteristic features are the following:---
The nasals are large, and so are the paroccipital processes. The
palate in front of the molars is not distinct- from the sides of the
skull, its edge gradually becoming rounded off above. It is also
very narrow. The .premaxillae are large in relation to the great
incisors. There is often a very much enlarged infra-orbital foramen
through which passes a part of the masseter muscle. The jugal
bone lies in the middle of the zygomatic arch, which is complete
and enormously enlarged in the Spotted Cavy (jCoelof/enys pac<z).
As in many Marsupials, the jugal bone sometimes extends back-
wards to the glenoid cavity, where the lower jaw articulates. It is
usually said with an absolute want of accuracy that the cerebral
hemispheres of the Rodents are smooth and without convolutions.
This error has been, repeated again and again in text-books. As
a matter of fact the cerebral hemispheres of many forms are quite
well convoluted,3 the degree of furrowing corresponding, as in so
many groups of mammals, with the size of the animal. This at
any rate is generally true, though the large Beaver with its scant
convolutions is an exception. The smaller forms., such as M^usf
Sciurus, Dipus, and Cricetus are quite smooth-brained. The best
furrowed brain of any Rodent which has been examined is that of
the huge Hydrochoerus. The Sylvian fissure is very generally
not pronounced ; but is particularly -well-marked in Lagostonvus.
In all, or in most, Rodents the hemispheres are separated by an
interval from the cerebellum, the optic lobes being visible between
the two.
The mouth cavity of this group of mammals is divided into
two chambers by a hairy ingrowth behind the incisors; this
arrangement is useful for animals which use their strong incisors
1 Phil. Trans. 1850, pt. ii. p. 529,                   s Seen, however, in Cha*tomy$.
* See Beddard, JPr<w. &v<rl. jSoc. 1892, p, 596, and Gervals, Jbum. Zvol. L 1872,
p. 450.